Dihydropyrones are important as intermediate products in drug synthesis. In particular, 5,6-dihydro-4-hydroxy-6-phenethyl-6-propyl-2H-pyran-2-one is an important intermediate product in the synthesis of tipranavir, an HIV protease inhibitor. The compounds of formula I and processes for preparing them are known from the prior art, e.g. from International Patent Application WO 98/19997 and from the “Journal of Medicinal Chemistry, 1998, Vol. 41, No. 18”. Recently, a process for preparing a racemic mixture of 5,6-dihydro-4-hydroxy-6-phenethyl-6-propyl-2H-pyran-2-one has been described, which comprises in step a) reacting a dianion of methylacetoacetate with 1-phenyl-3-hexanone and in a subsequent step b) cyclising the resulting β-ketoester by alkaline hydrolysis followed by acidification. This process is carried out discontinuously and achieves a 72% yield.
The problem of the invention is therefore to provide a process which enables dihydropyranones to be prepared with a high degree of purity and in a significantly improved yield compared with the prior art.